Patients with recurrent glioblastoma who are planned to receive a second course of radiation are to be included into this monocentric cohort trial. Due to multiple pre-treatments simultaneous combined positron emission tomography (PET) with O-(2-[18F]fluoroethyl)-l-tyrosine (FET) as well as magnetic resonance imaging (MRI) is used for treatment planning and follow-up imaging as it allows for a better distinction between treatment-related changes and viable tumor tissue.

For glioblastoma (GBM) patients it has been proven that a [18F]FET-PET scan is very helpful especially in target volume definition and after the treatment, in turn, the combination of MRI and [18F]FET-PET is diagnostically most useful to distinguish between radiation necrosis and a real progressive disease.

The response to therapy is based on the newly formulated Revised Assessment in Neuro-Oncology (RANO) criteria. Kinetic and static [18F]FET-PET scans are useful to supplement this modality and its own prognostic value concerning relapsing patients will be examined.

The special feature of this study is the use of both modalities in parallel, allowing simultaneous acquisition of morphological changes, functional and molecular imaging.

Secondary methodological issues are dealt with, such as the relationship between contrast uptake, perfusion and [18F]FET uptake. In this regard, the hybrid imaging may serve for hypothesis generation, as in parallel in a unique way of contrast enhancement and tracer kinetics can be investigated (simultaneous contrast-enhanced analysis and tracer application).

In particular, FET kinetics are examined in more detail (for example, differences between increasing and decreasing kinetics) to find ways of how to use certain MRI sequences for better visualization of viable tumor tissue and vice versa .

Inclusion Criteria:

• 18 - 75 years old
• histologically or magnetic resonance imaging proven recurrent glioblastoma
• re-irradiation possible
• willing and able to give free informed consent

Exclusion Criteria:

• not willing or able to give free informed consent
• pregnancy
• claustrophobia
• metallic objects or implanted medical devices in the body

Patients with recurrent glioblastoma who are planned to receive a re-irradiation